The present invention relates to a method for the biological processing of organic substances and more particularly for anaerobic biological hydrolysis for subsequent biomethanization and an apparatus for the performance of the method.
Biomethanization of complex organic materials is performed by the interplay of essentially three microorganism groups, that is to say:
1. Hydrolytic fermentative bacteria PA1 2. Hydrogen producing acetogenic bacteria. PA1 3. Methanogenic bacteria which consume hydrogen and acetate. PA1 1. An increase in the supply of solids (increase in load). PA1 2. Reduction in the return of the methanization output material. PA1 3. Modification of the separating effect in the solids/liquid separation stage. PA1 1. Increasing the return of methanization output material. PA1 2. Reduction the supply of solids.
In this respect the products of the first group are processed by the second microorganism group and the products of the second group are processed by the third group. As a result the main products, methane and carbon dioxide, are produced from complex biopolymers.
Fermentative microorganisms preferentially utilize simple organic substances such as glucose or cellobiose in order to obtain energy, for such substances are dissolved and may therefore be directly resorbed. Under anaerobic conditions in such a case the products of fermentation are mainly organic acids or alcohols.
On the other hand complex organic substances such as for instance cellulose are not directly resorbed. They have to be firstly hydrolyzed to resorbable fragments. For this purpose hydrolytically active microorganisms excrete specific exoenzymes, which degrade the biopolymers. The products of such decomposition are then fermented in the anaerobic environment to give organic acids or alcohols.
Since the fermentation of dissolved substances is substantially more favorable for the microorganisms energetically, they only form hydrolytic exoenzymes in the absence of dissolved fermentable substances (Buchhoiz, K. and H.-J Arntz (1988) "Gewinnung von Enzymen durch anaerobe Fermentation yon Rubenpre.beta.schnitzeln, Zuckerindustrie 113(1988), p. 204-208). This means that in the case of a substrate mixture of dissolved and undissolved substances it is firstly the dissolved substances which are substantially fermented, before the biopolymers are hydrolyzed and fermented.
In the case of an enrichment of the products in the medium there is an inhibition of the fermentation process. Such product-entailed inhibition is made more intense by the drop in the pH value due to the acids formed. It is more particularly in the case of pH values under 6 that owing to low dissociation of the organic acids formed a concentration inhibiting level is reached very quickly.
The activity of the hydrolytic enzymes is furthermore very much influenced by the pH value in the medium. Most hydrolytic enzymes of anaerobic microorganisms show maximum activity in a pH range of 6 to a little over 7(see Rodgriguez, H., Volfova, O. and Klyosov, A.: Characterization of the cellulose complex from Cellulomonas grown on bagasse pitch, App. Mircobiol. Biotechnolo. 28(1988), p, 394-397).
In order to counteract an enrichment of the acids formed and a drop in the pH value, it is necessary for the acids to be eliminated. Unlike the case of a single stage conduct of the method, in the case of which they are immediately methanized in the same reactor, in the case of multistage methods the acids produced in the hydrolysis and acidification stage have to be systematically removed and passed on to the methanization stage. This disadvantage of the multi-stage method is however countered by its higher efficiency (Bally, J. E. and Ollis, D. F.: Biochemical Engineering Fundamentals (McGraw-Hill, N.Y. 1977).
According to Noike (Noike, T., et al.: Characteristics of carbohydrate degradation and the rate-limiting step in anaerobic digestion, Biotechnology and Bioengineering, 27(1985), p. 1482-1489) in the case of the biomethanization of organic solids the anaerobic hydrolysis of the solids is the rate setting step. By setting to optimum conditions of the environment it is possible to increase the rate of conversion of the hydrolytic microorganism and consequently to accelerate the rate limiting degradation step. For many anaerobic hydrolytic microorganisms the optimum pH value is on the acid side.
For the biomethanization of the products of hydrolysis a neutral pH value is however more suitable. In the acidic range the material conversion of methan-ogenic populations decreases to a greater extent. As a result in the case of pH values, which are optimum for hydrolysis, the methanization of the products of hydrolysis is the rate limiting step. By separation of the two degradation steps by a dual-stage conduct of the method optimum basic conditions are obtained for both degradation steps.
In the case of continuous supply of complex substrates such as for instance mixtures of refuse, to the acidification stage, owing to non-interrupted supply of readily fermentable substances (which are generally in solution) hydrolysis of biopolymers is inhibited. Performance of the method in two stages with continuous feed of substrate and with acidification followed by hydrolysis suppresses such inhibition. The dissolved and readily fermentable substances are acidified in the first stage and only solids are added in the hydrolysis stage. Owing to this selection pressure a very active, hydrolytic population is established in the second stage with the result that the degradation of solids is increased.
Gonzales and co-workers (Gonzales, G., Caminal, G., De Mas, C., and Lopez-Santin, J.: A kinetic model for pretreated wheat straw saccharification by cellulase, J. Chem. Tech. Biotechnol. (1989) p. 275) showed that enzyme reactions for the degradation of cellulose may be described as Michaelis-Menten reactions. This means that high substrate concentrations have a favorable effect on hydrolysis. However with an increased concentration of solids the rheology in the reactor changes and owing to a limitation in transport the reaction rates decrease. Therefore for the hydrolysis of solids there is an optimum concentration thereof dependent on the respective substrate mixture.
However so far no such optimized methods have been described. In the case of the method described in the U.S. Pat. No. 4,781,836 for the biomethanization of organic substances with two processing stages the acidification of the dissolved components and the hydrolysis of the undissolved substance takes place in one and the same reactor in an environment which is not optimum for the processes performed therein. Furthermore the water circuits for the two reactors are completely separate from each other. This separation is produced by a combination of filters for solids/liquid separation and ion exchangers for the removal and transfer of the dissolved polar substances. Such a conduct of the method is unsuitable for the treatment of mixtures of complex substances containing solids, because suitable filters are either unable to deal with the acidified mixture or the filtrate contains too much solids to be dealt with by an ion exchanger. The European patent publication 89 890 162.4 A3 describes a method for increasing methane yield in the case of the fermentation of municipal organic waste using two separate stages, and two mixed fermentation stage involved therewith in a first and a second reactor, in the case of which respectively in the two reactors there is an acidification and methanization of the municipal organic waste. In the case of these methods as well the acidification of the dissolved components and the hydrolysis of the undissolved substances takes place in one and the same reactor under conditions which are not optimum.
Buchholz (1986), Gijzen and Zwart (Buchholz, K., Arntz, H.-J., Pelligrini A. and Stoppok E: Untersuchung zur Bildung von Biogas aus Rubenpre.beta.schnitzeln, Zuckerindustrie 111(1986), p. 837-844; Gijzen, H. J. et al.: High-rate two-phase process for the anaerobic degradation of cellulose employing rumen microorganisms for an efficient acidogenesis, biotechnology and bioengineering 31(1988), p, 418-425; Zwart, K. B. et al.: Anaerobic digestion of a cellulosic fraction of domestic refuse by a two-phase rumen derived process, Biotechnology and Bioengineering 32(1988) p. 724-729) describe in their publications a two-stage process for the anaerobic fermentation of organic solids. In the case of this method as well the acidification of the dissolved components and the hydrolysis of the undissolved substances take place in one reactor.
This joint acidification and hydrolysis involves the disadvantage that the formation of the hydrolytic exoenzymes is suppressed until the dissolved and readily fermentable substances are completely acidified. In the case of a continuous supply of substrate an equilibrium concentration of the non-acidified and readily fermentable becomes established in the reactor dependent on the conversion rates. The result of this is that the formation of the exoenzymes and therefore the hydrolysis of solids is inhibited.
It is only with a discontinuous feed to the first reactor that in the case of this two-stage method it is possible to reduce the concentration of readily fermentable substances at times to such low values that satisfactory hydrolysis of the solids which are difficult to degrade may be ensured.
In the publication of Hack, P. J. F. M. and Brinkmann, J. A.: New Process for High Performance Digestion, International Symposium on Anaerobic Digestion of Solid Waste, Venice 14-17.4.92, a three-stage conduct of the process with the steps of acidification, hydrolysis and methanization is proposed, the individual method steps being performed in spatially separated reactors and in which the return of the material from the methanization stage is utilized for control of the pH value and the concentration of solids. In this method, which in the following is termed the PAQUES method, after mechanical pretreatment the solids are hydrolyzed and acidified in a reactor 1 (prethane reactor), the solids fractions susceptible of rapid degradation passing into solution. The resulting slurry is separated and most of the solids fraction is returned back to the prethane reactor for further hydrolysis. Only a minor quantity of the solids is transferred to the second reactor (RUDAD reactor).
In this second reactor it is more particularly ciliata and anaerobic fungi which serve to hydrolyse the solids fraction and cellulose and other fibrous compounds are somewhat acidified. The final products of this hydrolysis method are more particularly volatile fatty acids. The non-degradable solids are removed from the RUDAD reactor.
In a third stage and in a third reactor the liquid fraction from the reactor 1 and the hydrolysis products from the reactor 2 are methanized.
The anaerobic material from the third reactor is employed in the prethane reactor and in the RUDAD reactor for dilution and for pH value control.
This PAQUES method suffers from the disadvantage that the three parameters which are more particularly relevant for the process, that is to say control of the pH value, the concentration of solids in reactor 2 and the residence time of the solids in reactor 2 are only able to be changed interdependently so that an arbitrary or systematic modification of all three parameters for the control of the rate of hydrolysis of solids is substantially impaired. A further disadvantage of the PAQUES method is the return of the greater part of the solids feed into the reactor 1 (the prethane reactor) after separation. This conduct of the method among other things necessitates a separation of the feeds into two part feeds and consequently more complex equipment. A further disadvantage is the degradation of the solids by ciliata and anaerobic fungi in reactor 2. Because such microorganisms are normally not present in the solids so processed, it is necessary for the material to be introduced into the reactor 2 with a special inoculating sludge so that the method becomes more expensive.